The present invention is directed at a process for catalytic cracking of hydrocarbon feedstocks. More specifically, the present invention is directed at a method for improving the octane number of feedstocks processed by catalytic cracking.
In the catalytic cracking of hydrocarbon feedstocks the feedstock is cracked into lower molecular weight products. One of the most important factors in determining catalytic cracking conditions is the octane number of the cracked product. One method of improving the octane number of the cracked product has been to use relatively expensive, specially formulated high octane cracking catalysts. However, the use of these catalysts is not advantageous in many instances, particularly where the feedstocks contain significant concentrations of metals, such as nickel, vanadium and/or iron. These metal contaminants become deposited on the cracking catalyst promoting excessive hydrogen and coke makes. Producing a high octane cracked product often has necessitated the frequent regeneration and/or replacement of the cracking catalyst.
Previously, it has been noted that the presence of metal contaminants, such as nickel, iron, and vanadium, on cracking catalyst may operate to improve the octane number of the cracked product. U.S. Pat. No. 4,200,520 describes a method for improving the octane number of cracked product by maintaining the metals content on the catalyst within the range of about 1,500 to 6,000 parts per million by weight (wppm), preferably from about 2,500 to about 4,000 wppm of equivalent nickel. The desired metals level is achieved by adding a metals-containing heavy feedstock intermittently or continuously with the gas oil. This patent also suggests maintaining the metals level within the predetermined limits by withdrawing high metals-containing catalyst from the system and adding low metals-containing catalyst to the cracking zone. However, adding metal-containing feeds may result in a large number of active metal sites which contribute to excess hydrogen and coke production. U.S. Pat. No. 3,718,553 also discloses that the octane number of a cracked feedstock may be improved by regulating the metals content on the feed. This patent discloses controlling the amount of nickel, iron, and/or vanadium on the catalyst within the range of about 100 to about 1,000 wppm by preimpregnating the catalyst within the desired amount and type of metal.
However, it also has been found that the presence on cracking catalyst of metal contaminants, such as nickel, vanadium, and iron, may lead to excessive hydrogen and coke makes. Several patents have been issued which disclose methods for reducing the detrimental effects of metal contaminants on cracking catalyst. U.S. Pat. Nos. 4,280,895 and 4,280,896 disclose that cracking catalyst can be passivated by passing the catalyst through a reducing zone having a reducing atmosphere therein maintained at an elevated temperature for a period of time ranging from about 30 seconds to 30 minutes. These patents also disclose that selected metal contaminants may be added to the cracking system to improve the degree of passivation. U.S. Pat. No. 4,298,459 describes a process for cracking a metals containing feedstock where the cracking catalyst is subjected to alternate exposures of up to 30 minutes in an oxidizing zone and in a reducing zone maintained at an elevated temperature to thereby reduce the hydrogen and coke makes. U.S. Pat. Nos. 4,268,416; 4,361,496; 4,364,848; and 4,382,015; European Patent Publication No. 52,356; and PCT Patent Publication No. WO/04063 all describe methods for passivating cracking catalyst in which metal contaminated cracking catalyst is contacted with a reducing gas at elevated temperatures to passivate the catalyst. However, these publications do not disclose a method for increasing the octane rating of the cracked product.
It is desirable to provide a process which would permit the production of a cracked product having a relatively high octane number without the production of excessive hydrogen and coke.
It also is desirable to provide a process in which a high octane cracked product is produced without the use of significant quantities of relatively expensive cracking catalyst.
It also is desirable to provide a process in which equilibrium catalyst which had been removed from cracking units may be reused.
The subject invention is directed at a process for improving the octane number of cracked product by maintaining the metals content at a predetermined level and by passing the catalyst which has been regenerated from the regeneration zone through a passivation zone prior to its return to the cracking zone.